Mysql中的CHECK约束特性详解

CHECK (expr)

[CONSTRAINT [symbol]] CHECK (expr) [[NOT] ENFORCED]

CREATE TABLE t1
(
  CHECK (c1 <> c2),
  c1 INT CHECK (c1 > 10),
  c2 INT CONSTRAINT c2_positive CHECK (c2 > 0),
  c3 INT CHECK (c3 < 100),
  CONSTRAINT c1_nonzero CHECK (c1 <> 0),
  CHECK (c1 > c3)
);

mysql> SHOW CREATE TABLE t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `c1` int(11) DEFAULT NULL,
  `c2` int(11) DEFAULT NULL,
  `c3` int(11) DEFAULT NULL,
  CONSTRAINT `c1_nonzero` CHECK ((`c1` <> 0)),
  CONSTRAINT `c2_positive` CHECK ((`c2` > 0)),
  CONSTRAINT `t1_chk_1` CHECK ((`c1` <> `c2`)),
  CONSTRAINT `t1_chk_2` CHECK ((`c1` > 10)),
  CONSTRAINT `t1_chk_3` CHECK ((`c3` < 100)),
  CONSTRAINT `t1_chk_4` CHECK ((`c1` > `c3`))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

CREATE TABLE Departments (
    ID int NOT NULL,
    PID int NOT NULL,
    Name varchar(255) NOT NULL Default '',
    CHECK (ID>=1)
);
-- add check separately
ALTER TABLE Departments
ADD CONSTRAINT CHK_PID CHECK (ID>=1 AND PID >=0);
-- remove check
ALTER TABLE Departments
DROP CHECK CHK_PID;

CREATE TABLE IF NOT EXISTS `department` (
  `id` int NOT NULL AUTO_INCREMENT,
  `pid` int COMMENT 'parent id',
  `name` varchar(100) NOT NULL,
  PRIMARY KEY (`id`)
  ) ENGINE = InnoDB;


CREATE TRIGGER pid_insert_check 
BEFORE INSERT ON department 
FOR EACH ROW 
BEGIN
  IF (NEW.pid <> 0 AND NEW.pid NOT IN (select id from department)) THEN
    signal sqlstate '45000'
    set message_text = 'department parent id has to be chosen from id';
  END IF;
END


CREATE TRIGGER pid_delete_check 
BEFORE DELETE ON department 
FOR EACH ROW 
BEGIN
  IF (OLD.id < 0 OR OLD.id IN (select pid from department)) THEN
    signal sqlstate '45000'
    set message_text = 'department parent id has to be chosen from id';
  END IF;
END